Method for producing strain tolerant multifilamentary oxide superconducting wire

ABSTRACT

A strain tolerant multifilamentary wire capable of carrying superconducting currents is provided comprising a plurality of discontinuous filaments formed from a high temperature superconducting material. The discontinuous filaments have a length at least several orders of magnitude greater than the filament diameter and are sufficiently strong while in an amorphous state to withstand compaction. A normal metal is interposed between and binds the discontinuous filaments to form a normal metal matrix capable of withstanding heat treatment for converting the filaments to a superconducting state. The geometry of the filaments within the normal metal matrix provides substantial filament-to-filament overlap, and the normal metal is sufficiently thin to allow supercurrent transfer between the overlapped discontinuous filaments but is also sufficiently thick to provide strain relief to the filaments.

This invention was made with Government support under Contract No.W-7405-ENG-82 awarded by the U.S. Department of Energy. The Governmenthas certain right in the invention.

This is a divisional of copending application Ser. No. 07/651,551, filedon Feb. 6, 1991, now U.S. Pat. No. 5,189,260.

FIELD OF THE INVENTION

The present invention relates to superconductors, and more particularlyto strain tolerant superconducting wires and a method of preparing thesesuperconducting wires.

BACKGROUND OF THE INVENTION

The phenomenon of superconductivity which many metals exhibit at lowtemperatures is of great scientific and commercial value since itpermits various high powered devices to operate with minimal losses ofelectrical power. Superconducting devices can be used in such commercialapplications as motors, generators, transformers, power lines, medicalimaging systems, and large scale supermagnets. Several metal alloys havebeen discovered which exhibit superconducting properties and have beenused to produce multifilament superconducting composites. Thesesuperconducting materials include Nb₃ Sn, V₃ Ga, Nb-Ti, and Nb₃ Al.These superconducting materials are typically combined with a normalconductive metal and then formed into long, thin filaments to produce amultifilament superconducting/normal metal composite.

One method for preparing a multifilament superconducting wire, theso-called "bronze process") having a superconducting material such asNb₃ Sn, consists of drilling a plurality of evenly spaced holes in acopper or bronze billet, inserting niobium rods in each hole, andextruding and drawing the billet in several steps until the niobium rodsare reduced to the desired filament size. The wire is then coated withtin and heated to react the tin and niobium in order to form the Nb₃ Snsuperconducting material. However, this process is expensive andexacting and generally limits the filament size to filaments larger than2 μm in diameter.

Another method for preparing a multifilamentary superconducting wire,the so-called in-situ process, involves casting large billets of Cu andNb together in a consumable arc casting process. This produces a castingin which a dense array of Nb dendrites about 6 μm in diameter aredispersed in a Cu matrix. This billet is drawn to wire, coated with Sn,and heat treated to transform the Nb to Nb₃ Sn. These types of compositesuperconducting wires are known as in-situ composites because thesuperconducting filaments are formed in place during the process ofpreparing the multifilament superconducting composite.

Thus, neither of the prior art methods of forming superconducting wiresdescribed above separately prepare the filaments of the superconductingmaterial prior to combining them with a normal metal material. Rather,as described above, the filaments are prepared during the castingprocess.

As stated above, multifilamentary superconducting composites have playeda central role in the development of conductors for commercialapplications such as large scale magnets because they are more stablemagnetically than monolithic tapes and they are far easier to handle inthe process of winding the magnet. The fabrication of these compositesfor high critical temperature materials such as those set forth above,however, has been problematic because the materials are so brittle andthe chemistry so complicated.

Advances recently have been made in the development of high temperaturesuperconducting materials based on copper-bearing oxides such as Y₁ Ba₂Cu₃ O₇ and various compositions of Bi-Sr-Ca-Cu-O. These materials havebeen processed using a wide variety of techniques in an attempt toproduce useful engineering devices. Some of the processing techniquesused include plasma spraying, sputtering, and laser-heated pedestalgrowth. However, because of the complexity of these processingtechniques, none have been found feasible for use on a mass productionbasis.

Other methods for processing these materials in filament form have beendeveloped which are more feasible for use on a mass production basis.These methods include a pendant drop melt extraction process and a gasjet fiberization process. The gas jet fiberization process for makingamorphous Bi-Sr-Ca-Cu-O fibers has thus far proved to be the mostsuccessful. With the gas jet fiberization process, liquid drops ofBi-Sr-Ca-Cu-O are directed through a supersonic nozzle where a gasstream shapes and freezes the liquid into filaments approximately 1 cmlong having diameters that range from 0.5 to 3 μm. The resultingfilaments are in an amorphous state, and must be converted to asuperconducting state in a subsequent process in order to producemultifilament superconducting wires. As can be seen, the gas jetfiberization process is an ex-situ process, since the filaments areformed separately and must separately be combined with a normal metalmaterial subsequent to their formation.

Although superconducting filaments formed from Bi-Sr-Ca-Cu-O showpromise for use in multifilament superconducting wires, several problemsexist which must be overcome. For example, superconducting filamentscomposed of Bi-Sr-Ca-Cu-O cannot feasibly be produced with the types ofin-situ production methods discussed above. A significant reason forthis is the complex chemical composition of these filaments. Therefore,new methods must be developed for using these separately formedfilaments to produce superconducting wires. Further, with the gas jetfiberization and pendant drop melt extraction methods, the resultingfilaments are in an amorphous state and must be converted to thesuperconducting state by heat treatment. Control of the conversion ofthese amorphous filaments to the superconducting state is important,because the filaments can coarsen during heat treatment which destroysthe long slender aspect ratio of the filaments. Finally, because of thegeometry of the filaments, that is, their comparatively short length, onthe order of 1 cm, and their resulting discontinuous nature, it isdifficult to realize large supercurrents in a superconducting wirehaving only a relatively small number of these filaments. Thus, a methodof using a large number of these discontinuous filaments must bedeveloped which provides substantial filament-to-filament transfer ofsupercurrents among the discontinuous filaments.

In spite of these difficulties, the use of Bi-Sr-Ca-Cu-O filaments hasmany potential advantages. For example, with the gas jet fiberizationprocess, the resulting ex-situ filaments are amorphous, electricallyinsulating and relatively strong. The flexibility of these filaments,because they are in an amorphous state, allows them to better withstandmechanical processing to form microfilamentary superconducting wires.Further, the use of Bi-Sr-Ca-Cu-O filaments has great advantages overthe use of filaments formed from other superconducting materials such asthose set forth above. For example, the superconducting materialsdiscussed above exhibit poor mechanical properties, namely brittleness,and are not as reliable for use in commercial applications. As a result,these other superconducting materials are not as suitable for productionon an industrial scale. Therefore, if the problems discussed above canbe overcome, it may be possible to use the flexible Bi-Sr-Ca-Cu-Ofilaments for producing superconducting wires on a mass productionbasis.

SUMMARY OF THE INVENTION

In view of the foregoing, it is a general aim of the present inventionto provide a highly strain tolerant microfilamentary wire materialcapable of carrying superconducting currents and a process for makingit. In accomplishing that aim, it is an object of the present inventionto provide a microfilamentary superconducting wire formed from acomposite of a high temperature superconducting material and a normalconductive metal having a special geometry that permits high straintolerance.

An additional object of the present invention is to provide a method ofproducing a superconducting wire having such properties.

It is another object of the present invention to develop an ex-situmethod of producing semiconductor/normal metal composites using flexibleBi-Sr-Ca-Cu-O filaments.

Because Bi-Sr-Ca-Cu-O filaments are amorphous as formed, it is anadditional object of the present invention to develop a method oftransforming the filaments from the amorphous state to a superconductingstate. Also, because of the geometry of Bi-Sr-Ca-Cu-O filaments, in thatthey have a large length to diameter ratio but are discontinuous innature, it is yet another object of the present invention to develop amethod of combining a large number of these filaments into a normalmetal material so that large supercurrents can be realized.

It is a feature of the superconducting wire of the present inventionthat the superconducting wire is made up of a plurality of discontinuoussuperconducting filaments of a high temperature superconductingmaterial, the filaments having a length at least several orders ofmagnitude greater than the filament diameter and being sufficientlystrong while in an amorphous state to withstand compaction. It is arelated feature that a very thin layer of normal metal is interposedbetween and binds the discontinuous plurality of filaments to form anormal metal matrix which is capable of withstanding heat treatment forconverting the amorphous filaments to a superconducting state.

It is another feature of the present invention that the geometry of theplurality of filaments within the normal metal matrix providessubstantial filament-to-filament overlap to produce superconductingcurrent transfer among the plurality of filaments.

It is yet another feature of the superconducting wire of the presentinvention that the normal metal interposed between and binding theplurality of discontinuous filaments is sufficiently thin to allowsuperconducting current transfer between the overlapped filaments whilealso being sufficiently thick to provide strain relief to the filaments.An additional feature of the present invention is that when theamorphous filaments are converted to the superconducting state, thenormal metal matrix retains enough structural integrity to preserve thegeometry of the plurality of filaments in the superconducting wire.

In accordance with the strain tolerant microfilamentary superconductingwire of the present invention, a plurality of amorphous discontinuousfilaments are provided, the exterior surfaces of the filaments beingcoated with a thin layer of a normal conductive metal. The amorphousfilaments are formed from a high temperature superconducting material,and have a geometry in which the filament length is at least severalorders of magnitude greater than the filament diameter. The coatedamorphous filaments are then oriented such that the lengths of thefilaments lie generally along a desired axis of the superconductingwire, with the filament orientation producing substantialfilament-to-filament overlap. Then, the oriented coated filaments arecompacted to form an elongate composite in which the overlapped coatedfilaments are pressed into contact with each other. The composite isthen heat treated to crystallize the amorphous filaments while leaving anormal metal interface between the crystallized filaments. This heattreatment preserves the normal metal coating on the filaments to formthe normal metal interface in order to produce a superconducting wire inwhich the crystallized superconducting filaments are supported in anormal metal matrix. The normal metal matrix provides strain reliefbetween the crystallized filaments of the superconducting wire.

According to the present invention, a strain tolerant microfilamentarysuperconducting wire is provided comprising a plurality of discontinuousfilaments formed from a high temperature superconducting material. Thefilaments have a length at least several orders of magnitude greaterthan the filament diameter and are sufficiently strong while in anamorphous state to withstand compaction. The superconducting wirefurther comprises a normal metal interposed between and binding thediscontinuous filaments to form a normal metal matrix which is capableof withstanding heat treatment when the filaments are converted to asuperconducting state. The geometry of the filaments within the normalmetal matrix provides substantial filament-to-filament overlap.Additionally, the normal metal of the normal metal matrix issufficiently thin to allow supercurrent transfer between the overlappeddiscontinuous filaments while also being sufficiently thick to providestrain relief to the filaments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing a single filament of thesuperconducting material used in the superconducting wire of the presentinvention;

FIG. 2 is a perspective view showing the single filament of FIG. 1coated with a normal metal material;

FIG. 3 is a perspective view showing a plurality of the coated filamentsof FIG. 2 formed into a yarn;

FIG. 4 is a perspective view showing three coated filaments which form aportion of the yarn shown in FIG. 3;

FIG. 5 is a perspective view showing the plurality of coatedsuperconducting filaments as shown in FIG. 3 inserted in a normal metaltube and pressed into a dense microfilamentary composite; and

FIG. 6 is a partial magnified front view of the composite of FIG. 5showing the plurality of filaments in the superconducting state afterheat treatment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

While the invention will be described in connection with a preferredembodiment, there is no intent to limit the invention to thisembodiment. On the contrary, the intent is to cover all alternatives,modifications, and equivalents included within the spirit and scope ofthe invention as defined by the appended claims.

Turning now to the drawings, FIG. 1 is a perspective view of a singleamorphous filament 10 formed from a high temperature superconductingmaterial used to form the highly strain tolerant superconducting wirematerial of the present invention. In the preferred embodiment of thepresent invention, superconducting filaments similar to that shown inFIG.1 are formed from the high temperature superconducting materialBi-Sr-Ca-Cu-O with the gas jet fiberization method briefly describedaboveand described in detail in S. E. LeBeau, J. Righi, J. E. Ostenson,S. C. Sanders, and D. K. Finnemore, "Preparation of SuperconductingBi-Sr-Ca-Cu-O Fibers" Appl Phys Lett 55, 292 (1989). It should be notedthat various compositions of this superconducting material can be useddepending on the particular performance characteristics desired. Forexample, a filament formed from the Bi₂ -Sr₂ -Ca₂ -Cu₃-O₁₀₋₈ compoundhas a higher transition temperature and better performance at hightemperatures in the range of 78K. However, the Bi₂ -Sr₂ -Ca₁ -Cu₂ -O₈₋₈has a lower transition temperature but performs better at 4.2K.Additionally, it may be desirableto replace some of the Bi with Pb inorder to significantly lower the temperature needed to crystallize thefilaments. Thus, the strain tolerantsuperconducting wire of the presentinvention can be formed from any of these various compositions ofsuperconducting material with the process described in detail below.

In the preferred embodiment, in order to fabricate the microfilamentarystrain tolerant superconducting wire of the present invention, anex-situ method of formation is used. First, a plurality of amorphousfilaments of (Bi-Pb)-Sr-Ca-Cu-O of the kind shown in FIG. 1 are cleanedto remove any beads, shot, chaff and surface contamination in anultrasonic bath containing isopropanol. The filaments are then dried inair and placed on a stainless steel pan in an evaporator. Theseamorphous filaments should have a large length to diameter ratio,preferably at least several orders of magnitude, and most preferably onthe order of 10,000:1. Although the filaments have a large length todiameter ratio, the filaments are discontinuous in nature. That is, thefilaments have a relatively short length, on the order of 1 cm, with adiameter of about 1 μm. Therefore,it is necessary to provide anextremely large number of these discontinuousfilaments in a singlesuperconducting wire. For example, in the preferred embodiment, a crosssection of a 1 mm diameter superconducting wire would containapproximately one million discontinuous filaments.

After the filaments are cleaned, they are coated with a normal metalmaterial by thermal evaporation, chemical vapor deposition or othersimilar processes. In the preferred embodiment, the filaments are coatedwith Ag which is evaporated from a tungsten coil, with the first coatinghaving a thickness of about 0.1 μm. The filaments are then intermixedand another similar evaporation of Ag is made. This process is repeatedseveral times until there is approximately 0.5 μm of Ag on thefilaments. FIG. 2 shows a coated filament 14 comprising amorphousfilament10 coated with Ag coating 12.

In order to produce substantial filament-to-filament overlap so thatsupercurrents can transfer between the plurality of filaments when theyare converted to the superconducting state, the plurality of coatedfilaments 14 are oriented so that the lengths of the filaments liegenerally along a desired axis of the superconducting wire. The step oforienting produces a configuration in which a large number of filamentsare formed into a bundle or yarn, with the plurality of filamentsclosely interleaving and overlapping each other Thus, the term "yarn"when used inthe description of the present invention, means a pluralityof filaments which are oriented and overlapped as described herein toachieve the filament-to-filament overlap needed for supercurrenttransfer. In order toorient the filaments, coated filaments 14 areplaced in a steam of freon and floated or propelled against a barrier toalign the long axis of all the plurality of coated filaments 14 inapproximately the same direction. As shown in FIG. 3, the orienting stepproduces a yarn 16 of coated filaments 14 of the type shown in FIG. 2.Yarn 16 is then dried in flowingair.

FIG. 4 is shown to demonstrate the substantial filament-to-filamentoverlapthat exists between the plurality of filaments which form yarn16. FIG. 4 is a magnified view showing three coated filaments 14 whichform a portionof yarn 16 shown in FIG. 3. As stated above, currents musttransfer betweenthe plurality of overlapped filaments due to theirdiscontinuous nature. Inorder to demonstrate this filament-to-filamentcurrent transfer, FIG. 4 shows currents 18 which flow along the axes ofthe filaments, and couplingcurrents 20 which transfer between thediscontinuous filaments. FIG. 4 alsoshows an overlap area or crossoverarea 22 which represents the overlap area for transfer of couplingcurrents 20 between the filaments, and a current transport area 24 whichrepresents the area that defines the flow of currents 18 within thefilaments.

The term "substantial" when used to describe the amount offilament-to-filament overlap, means that the overlapped and closelyinterleaved dispersal of the filaments produced in the orienting step,along with the large length to diameter ratio of the filaments, providesan overlap between the plurality of filaments which encompasses nearlytheentire area of each of the filaments. In the preferred embodiment ofthe present invention, the crossover area 22 for filament-to-filamentcurrent transfer is at least several orders of magnitude, and mostpreferably about 10,000 times larger than current transport area 24.Thus, in keepingwith the invention, this enormous overlap area allowsfor supercurrent transfer between the plurality of discontinuousfilaments so that superconducting currents can be realized in the straintolerant microfilamentary superconducting wire of the present invention.

After the orienting step, yarn 16 of coated filaments 14 is compacted toform an elongate composite in order to closely press coated filaments 14into contact with each other. In one example according to the presentinvention, a silver foil is rolled around yarn 16 similar to the way inwhich a cigarette is rolled. In alternative examples, yarn 16 of coatedfilaments 14 may be mixed with a silver-based powder in varying degreesdepending on the specific requirements of the superconducting wire.After yarn 16 is rolled in a silver foil, it is placed in a silver tube.The ends of the tube are sealed in a vacuum, and then the silver tubecontaining the filaments is compacted by cold isostatic pressing orswaging to form an elongate composite.

FIG. 5 shows a plurality of coated filaments 14 placed within a silvertube26 which has been compacted to form an elongate composite 28. As canbe seen in FIG. 5, the plurality of coated filaments 14 are pressed intoclose contact with each other so that supercurrents can transfer betweenthe overlapped filaments. It should be noted that FIG. 5 is not only anenlarged view, but is also simplified to show only a relatively smallnumber of filaments, recalling that in a practical embodiment of thepresent invention there can be approximately one million filaments in across section of a 1 mm diameter wire.

The plurality of coated filaments 14 within elongate composite 28 are inanamorphous state, and must be converted to the superconducting state bya heat treatment process. As stated above, control of this conversionprocess is very important, because the filaments can coarsen during heattreatment which destroys the long slender aspect ratio of the filaments.Additionally, the temperatures used to convert the amorphous filamentsto the superconducting phase must be low enough to preserve the normalmetal coating which forms a normal metal matrix surrounding theplurality of filaments. For example, if the temperatures used in theheat treatment process are too high, the normal metal matrix interposedbetween and binding the filaments can melt and flow, which destroys thecontinuity of the matrix. However, if successful, this heat treatmentprocess produces asuperconducting wire having a geometry in which theplurality of superconducting filaments are supported in a normal metalmatrix so that the normal metal matrix provides strain relief betweenthe relatively brittle superconducting filaments.

In the preferred practice of the invention, in order to convert theamorphous filaments to the superconducting state, the temperature ofelongate composite 28 is ramped rapidly to a temperature just above themelting point of the superconducting material and held for a specifiedtime. This time may vary from several minutes to several hours dependingon the composition of the superconducting material. However, it is ofgreat importance that the temperature remain low enough to preserve thenormal metal matrix of silver, so that the normal metal matrix retainssufficient structural integrity to preserve the geometry of theplurality of filaments in the superconducting wire. The exacttime-temperature profile and oxygen atmosphere for the heat treatmentdepends on the composition of the superconducting filaments and can bedetermined for anygiven composition by optimization procedures known tothose skilled in the art. It should be noted that the conversion of theamorphous filaments to the superconducting phase can be done by eitherraising the temperature and crystallizing the filaments without meltingthem, or by melting the filaments and refreezing the liquid to thedesired superconducting phase.

FIG. 6 is a partial magnified front view of a superconducting wire 34 ofthe present invention. Superconducting wire 34 is formed from elongatecomposite 28 shown in FIG. 4 subjected to heat treatment, and has aplurality of superconducting filaments 30 dispersed in a normal metalmatrix 32 composed of silver. As can be seen in FIG. 6, aftercrystallization, at least with some materials, the superconductingfilaments 30 exhibit a micro-ribbon-like shape due to the growthmorphology of the superconducting material.

Referring to FIG. 6, a superconducting wire according to the presentinvention is comprised of a plurality of discontinuous filaments formedfrom a high temperature superconducting material. As stated above, inthe preferred embodiment, the superconducting material is a compositionof thehigh temperature superconducting material (Bi-Pb)-Sr-Ca-Cu-O.These filaments must be sufficiently strong while in an amorphous stateto withstand mechanical compaction used to form the superconductingwire. Further, the filaments must have a length at least several ordersof magnitude greater than the filament diameter, preferably on the orderof 10,000:1.

Because of the extremely small dimensions of the superconducting wire ofthe present invention, FIG. 6 is a partial magnified schematic view ofthesuperconducting wire and is shown to identify the normal metalinterposed between the plurality of superconducting filaments. In actualpractice, because the normal metal coating is very thin, superconductingfilaments 30 are much closer together. As shown in FIG. 6,superconducting wire 34 comprises a normal metal which is interposedbetween and binds the plurality of superconducting filaments 30 to forma normal metal matrix 32. Normal metal matrix 32 must be capable ofwithstanding heat treatment for converting the originally amorphousfilaments to the superconducting state. In the preferred embodiment,normal metal matrix 32 is comprised ofsilver. However, in alternativeembodiments, copper, aluminum, lead, or gold can be used as normalconductive metal to form matrix 32.

Because the superconducting filaments are discontinuous, current mustflow through the normal metal to travel between filaments. Therefore, inorder to provide supercurrent transfer between the filaments, thegeometry of the filaments within the normal metal matrix must provide asubstantial overlap area between the filaments as described above andshown in FIG. 4.Further, the normal metal interposed between thefilaments must be of a minimal thickness in order to allow supercurrenttransfer between the overlapped filaments. The thickness of normal metalinterposed between thefilaments is highly significant, because thesuperconducting Cooper pairs, upon entering the normal metal material,will decay and revert into the normal state depending on the thicknessof the normal metal material. For example, a decay length is a measureof the length of penetration into thenormal metal at which Cooper pairswill revert into the normal state. At the first decay length, one-halfof the superconducting electrons will revert to the normal state.Similarly, at the second decay length, anotherone-half of the remainingsuperconducting electrons will revert to the normal state. In silver, adecay length is approximately 100-200 Å. Thus, if the normal metalinterposed between the filaments is overly thick, nearly all of thesuperconducting Cooper pairs of electrons will revert to the normalstate, and very little supercurrents will flow. However, because anextremely large number of electrons are present in thesuperconductingwire, and because of the large overlap area, the normal metal thicknesscan be several decay lengths thick.

In keeping with the present invention, the normal metal interposedbetween the plurality of filaments is sufficiently thin to allowsupercurrent transfer between the filaments, but is also sufficientlythick to provide strain relief to the filaments. In the preferredembodiment, the thicknessof the silver coating on the filaments isapproximately 0.05 to 0.5 μm. Thinner layers of silver or other normalmetal matrix material can be used, when desired, although decreasing thethickness of the matrix would most likely sacrifice some of the straintolerance of the superconducting wire. Additionally, the geometry of thefilaments within the normal metal matrix provides an enormousfilament-to-filament overlap area as shown in FIG. 4 and describedabove, so that supercurrents can be realized.

The microfilamentary superconducting wire of the present invention alsopermits large normal current flow when supercurrents are not realized.Because of the enormous overlap area between the filaments and thesufficiently thin normal metal matrix, a low effective electricalresistivity in the superconducting wire is provided, on the order of10⁻¹⁴ Ω-cm. Thus, even if supercurrents do not flow, the low effectiveelectrical resistivity in the superconducting wire of the presentinvention will permit large normal current flow.

Finally, in accordance with the present invention, the normal metalmatrix is capable of withstanding the heat treatment needed to convertthe filaments to the superconducting phase. Therefore, the normal metalmatrixretains sufficient structural integrity to provide significantstrain relief to the filaments, and to preserve the geometry of theplurality of filaments in the superconducting wire so that supercurrentswill flow.

In one example according to the invention, after the step of coating, ameasure of the distribution of filament plus Ag diameters showed most ofthe coated filaments 14 having diameters between 1 μm and 6 μm with apeak near 2 μm. Further, after yarn 16 of coated filaments 14 was formedas shown in FIG. 3, it was rolled in a silver foil about 50 μm thick andplaced in a silver tube which had an outer diameter of 3.3 mm and aninner diameter of 2.8 mm. The ends of the tube were sealed in a vacuumand cold isostatically pressed to 140 MPa at about 20° C. toformelongate composite 28 shown in FIG. 4. Elongate composite 28 was thenplaced in a furnace which was heated from 20° C. to 860° C. at a rate of50° C. per hour. The temperature was held at 860° C. for approximately0.5 hours, and then raised to 890° C. at a rate of 60° C. per hour andheld at 890° C. for oneminute. The temperature was then lowered from890° C. to 870°C. at a rate of 5° C. per hour and held at 870° C. forsixteen hours. Finally, the temperature was lowered to 20° C. at a rateof 50° C. per hour, and composite 28 was removed from the furnace.

The dimensions of the superconducting wire in this example included acrosssectional area of the silver tube of 0.092 cm², and a crosssectional area of the microfilamentary superconducting/normal metalcomposite of 0.014 cm². The critical current density, J_(c), measured atvarioustemperatures was found to be 1430, 710, 360, and 71 A/cm² at 10K,18K,28K, and 55K, respectively. At 4.2K, J_(c) was 7000 A/cm² in a zeromagnetic field, but dropped to 800 A/cm² at 0.5 T and then to 200 A/cm²at 16 T. In order to measure the strain tolerance of the superconductingwire of the present invention, the sample wire was bent around acylindrical mandrel and measured in place at 4.2K in zero magneticfield. Under these conditions, the sample exhibited critical currentdensities of 5000, 4900, and 4700 A/cm² at bending strains of0, 0.8% and1.1%, respectively. Subsequent samples of the superconducting wire ofthe present invention exhibited a critical current density of 5000A/cm²while under 1.2% bending strain at 4.2K in a zero magnetic field.

As is evident from the foregoing description, the present inventionprovides a highly strain tolerant superconducting wire material and aprocess for making it. The superconducting wire is formed from a hightemperature superconducting material, so that commercial applicationsusing the superconducting wire of the present invention can operate athigher temperatures than those applications using previous lowtemperaturesuperconductors. Further, as demonstrated above, thesuperconducting wire of the present invention can produce substantialsupercurrents while undersignificant amounts of strain.

We claim:
 1. A method of producing a strain tolerant multifilamentarywire capable of carrying superconducting currents comprising the stepsof:providing a plurality of amorphous filaments of a high Tc materialcapable of conducting superconducting currents when in the crystallinestate, the length of the filaments being at least several orders ofmagnitude greater than the filament diameter; coating the filaments witha thin layer of normal conductive metal on the exterior surfaces of thefilaments; orienting the coated filaments such that the length of thefilaments is generally along an elongate axis of the microfilamentarywire, the step of orienting producing substantial filament-to-filamentoverlap; compacting the oriented coated filaments to form an elongatecomposite in which the overlapped coated filaments are pressed intocontact with each other; and heat treating the composite to crystallizethe amorphous filaments while leaving a normal metal interface betweenthe crystallized filaments, the step of heat treating serving topreserve the normal metal coating on the filaments to form saidinterface to produce the multifilamentary wire capable of carryingsuperconducting currents in which the crystallized superconductingfilaments are supported in a normal metal matrix.
 2. The method of claim1 wherein the material used to form the amorphous filaments is the hightemperature superconducting material (Bi,Pb)-Sr-Ca-Cu-O.
 3. The methodof claim 1 wherein the step of processing comprises coating theamorphous filaments with a thin layer of a normal conductive metal bythermal evaporation or chemical vapor deposition to a thickness of about0.5 μm.
 4. The method of claim I further comprising distributing asilver based powder with the oriented coated filaments prior tocompacting the filaments.
 5. The method of claim 1 wherein the step oforienting the plurality of coated filaments comprises propelling thefilaments against a barrier in order to form the filaments into a yarnso that the lengths of the plurality of filaments lie generally along anelongate axis of the filamentary wire.
 6. The method of claim I whereinthe compacting step further comprises:placing the oriented coatedfilaments in a normal metal tube; and compacting the normal metal tubeby cold isostatic pressing or swaging to form an elongate composite. 7.The method of claim 1 wherein the step of heat treating the compositecomprises cycling the heat treating temperature in a range capable ofcrystallizing the amorphous filaments without melting the normal metalsuch that the normal metal matrix retains sufficient structuralintegrity to preserve the arrangement of the plurality of filaments inthe multifilamentary wire.
 8. The method of claim 1 wherein the normalconductive metal coating on the amorphous filaments is silver.
 9. Themethod of claim 8 wherein the temperature during the heat treating stepis sufficiently high to crystallize the amorphous filaments but is lowerthan the melting point of the silver coating.
 10. The method of claim 1wherein the normal metal matrix is sufficiently thin to allow fortransfer of superconducting currents between the overlapped filaments.